1. Field of the Invention
This invention relates to a process for the production of a glycol from an aldehyde feedstock, and more particularly, relates to an efficient electrochemical coupling of formaldehyde in neutral or acidic aqueous or aqueous-organic solutions at carbon-based electrodes to form ethylene glycol.
2. Description of the Prior Art
The formation of glycols through the mechanism of an electrochemical coupling of selected aldehydes and ketones is a generic reaction well documented in the prior art. The electrochemical reduction of formaldehyde, however, generally yields methanol, the product of a two-electron reduction, rather than ethylene glycol, the coupled product of a one-electron reduction. Only two references are believed to document the electrochemical coupling of formaldehyde to glycol, and the methods described in each of these reports are flawed in important respects. In J.Gen.Chem. USSR, 1973, 43, 2769, Tomilov and co-workers disclosed an attempt to couple formaldehyde to glycol at graphite electrodes in acidic solutions of KH.sub.2 PO.sub.4 in the presence of Hg(II) ions. However, the current efficiency reported in this system was only 25%, which is unacceptably low for commercial application. In addition, the presence of mercury in the reaction is particularly undesirable for public health and environmental concerns. More recently, in U.S. Pat. No. 4,270,992, Saito has reported that ethylene glycol can be formed at current efficiencies as high as 80% when formaldehyde is electrolyzed at a carbon type cathode in very alkaline solutions. The highest current efficiencies were reported in formalin solutions of NaOH and KOH, at a reaction temperature of 50.degree. C. This system, however, possesses at least two serious drawbacks. First, formaldehyde undergoes the well known base-catalyzed Cannizzaro reaction under the conditions present in Saito's electrolysis, in which two moles of formaldehyde yield one mole each of methanol and methyl formate, thus resulting in nonproductive consumption of formaldehyde. Second, ethylene glycol must be separated from an aqueous base solution. Separation of the desired product from the electrolyte is a complicating factor in any electrochemical synthesis, and this problem is aggravated where the product is less volatile than either the reactants or solvent, particularly since it is desirable to recycle the electrolyte and any unreacted starting materials in a simple and inexpensive manner. In Saito's system, the product glycol is less volatile than either the formaldehyde reactant or the water solvent, and such common separation techniques as distillation would entail a separation of glycol from solid NaOH or KOH.
It an object of this invention to provide an efficient electrolytic method for converting aldehydes, such as formaldehyde, to glycols, such as ethylene glycol, that does not have the selectivity affected by harmful competing side reactions and that is capable of producing product in high yield and selectivity.
It is another object of this invention to provide a process for producing ethylene glycol from formaldehyde in a simple manner with high current efficiencies and high product yields and selectivities in neutral or acidic aqueous or aqueous-organic cosolvent solutions.
It is still another object of this invention to provide a process wherein the glycol product can be easily and effectively separated from a reaction mixture containing a high boiling organic cosolvent.